Abstract

Surface passivation plays a dual role of protecting copper (Cu) from getting oxidized and reducing the surface roughness of Cu thin film and thus enables low-temperature Cu-Cu bonding. In this paper, we utilized an oxidation resistive, CMOS compatible copper-nickel-based alloy, constantan as a passivation layer, as it does not get oxidized at room temperature and is also CMOS damascene process compatible. Systematically optimized ultrathin, constantan (2 nm) on Cu surface not only protected Cu from oxidation but also reduced the surface roughness to about 0.7 nm, which led to Cu-Cu bonding at a temperature as low as 150 degrees C at 4 bar pressure. Cross-sectional TEM of bonded Cu layer reveals very significant interdiffusion and grain growth between the Cu films resulting in a zigzag bonding interface and the presence of Cu grains that extend across the entire bonding layer. Furthermore, reliability investigation of passivated Cu bonded structure was demonstrated using current stressing, temperature cycling test, and relative humidity test, which suggest excellent stable bonding without electrical performance degradation. This method of alloy-based passivation is confirmed to be effective and enable low temperature, low-pressure Cu-Cu wafer on wafer bonding for 3-D integration applications.